The present invention relates to apparatus for detecting indicia in webs or sheets, especially in moving webs. More particularly, the invention relates to improvements in apparatus for monitoring a running web of photographic paper or the like for the presence of indicia in the form of holes, notches, cutouts and like markers which are obtained by removing material from the web (for the sake of simplicity, such indicia will be called holes with the understanding that the term "holes" also embraces notches, perforations and like voids through which light is free to pass from the one side to the other side of the web).
Webs of exposed and developed photographic paper are provided with holes to denote successive prints. Such holes are monitored by a photoconductive detector system which is installed in a photographic processing laboratory and transmits signals to the transporting system for the web and/or to apparatus which process the web, particularly to a severing mechanism which subdivides the web into discrete prints. Such prints are assembled with corresponding customer films and are returned to dealers or directly to customers. Reference may be had to commonly owned U.S. Pat. No. 4,039,258 granted Aug. 2, 1977 to Hujer et al.
Certain presently known apparatus for detection of holes in running webs of photographic paper comprise a substantially U-shaped housing having two spaced-apart legs. The web is transported lengthwise through the gap between the legs of the housing. One leg contains a photoconductive detector and the other leg contains a light source which directs light against the detector. The detector transmits a signal when it is exposed to light which issues from the source and passes through a hole in that portion of the web which advances through the aforementioned gap. A drawback of such apparatus is that each leg of the housing must contain electric conductors which connect an energy source with the light source and which connect the detector with an amplifier or with another device for reception and processing of signals which are generated when the detector is exposed to light.
German Offenlegungsschrift No. 2,246,066 discloses an apparatus which employs two elongated filamentary light conductors disposed at one side of the path for the moving web. One end portion of each light conductor is immediately adjacent to the respective side of the web so that light which issues from the end portion of one light conductor is reflected by the moving web and enters the end portion of the other light conductor. The second end portion of the one light conductor is connected with a light source, and the second end portion of the other light conductor directs light against a detector. When the light which issues from the one conductor penetrates through a hole of the moving web, the detector transmits a signal which is indicative of the presence of a hole. The filaments of the light conductors are twisted so that the conductors form a cable of helically interlaced filaments. Such apparatus, too, exhibit serious drawbacks. Thus, the light conductors must consist of a high-quality material and they must be assembled and mounted with a high degree of precision. Furthermore, the range of such apparatus is very narrow, i.e., the apparatus can only detect holes of a given size and/or shape, and the holes must be located at a fixed distance from the margins of the running web.